1. Field of the Invention
The present invention relates to a color copier, printer, facsimile apparatus or similar color image forming apparatus of the type charging a photoconductive belt with a charger, exposing the charged surface of the belt with an exposing unit imagewise to thereby form a latent image thereon, developing the latent image with a developing unit to thereby form a toner image, repeating such a sequence of steps to form toner images of different colors on the belt one above the other and transferring the resulting composite color image to a recording medium with an image transferring unit, and a process cartridge therefore.
2. Description of the Background Art
Japanese patent laid-open publication No. 8-339110, for example, discloses a color image forming apparatus of the type forming toner images of different colors on a single photoconductive belt one above the other and transferring the resulting composite toner image to a recording medium. This type of color image forming apparatus, sometimes referred to as an image-on-image type color image forming apparatus, has a critical problem left unsolved, as will be described hereinafter.
Assume that a developing unit that forms a toner image on a photoconductive belt first is a first-color developing unit while developing units following it are a second-color and successive developing units. Then, the second-color and successive developing units sequentially form toner images of respective colors on the photoconductive belt over the toner image already formed by the first-color developing unit. At this instant, it is likely that part of toner, constituting the toner image formed on the belt first, is introduced into the second-color and successive developing units, disturbing the original colors of developers stored in the second and successive developing units. This degrades the quality of a composite toner image formed on the belt to a critical degree.